Circuit breakers for low-voltage circuits are generally known. They are available in different designs which are referred to as molded case circuit breakers, air circuit breakers etc. A low-voltage circuit is used to mean a circuit or an electrical installation for voltages of up to 1000 V AC or 1500 V DC.
Circuit breakers monitor the electrical circuit for the presence of current or other electrical conditions. Circuit breakers interrupt the electrical circuit, in particular in the case of short circuits or overcurrents which exceed previously stipulated current values.
Different measuring transducers are used to measure the electrical current of an electrical conductor. These measuring transducers generally provide a variable proportional to the electrical current of the electrical conductor or an equivalent.
As measuring transducers for the electrical current, use is made, for example, of transformers or Rogowski transducers. The latter provide a voltage or a voltage signal which corresponds to the derivative of the electrical current I of the electrical conductor.
  u  =      M    *                  d        ⁢                                  ⁢        t                    d        ⁢                                  ⁢        t            
Rogowski transducers (air coil without an iron core) have the advantage of a very large current and frequency measurement range and are therefore preferably used as measuring transducers or current sensors in circuit breakers for low voltage.
The constant M, the so-called mutual inductance, is dependent on the physical structure of the Rogowski transducer, that is to say dependent on the number of turns, cross-sectional area etc.
The transducer voltage must be integrated in order to obtain a signal proportional to the conductor current. Integration is required, in particular, when non-sinusoidal currents are present or when an active power or a reactive power is intended to be determined from the measured current and the measured voltage since the phase shift is important here. The integration is carried out with the aid of an analog integrator which is passive in most cases.
An analog integrator is implemented using real components afflicted with loss and production process tolerances. In particular, the actual integration capacitor has a great influence on the overall behavior of the integrator circuit.
The integration circuit has a considerable influence on the measurement accuracy of the corresponding measuring circuit. Temperature dependence, in particular, is a problem. In order to maintain narrow tolerance bands, the measuring circuits are calibrated, that is to say factors which are used to continuously correct the measured values are determined. The properties of the components of the integration circuit change as a result of the influence of the temperature, which results in a deviation of the measured values. This effect can be reduced by way of a complicated temperature-dependent method for correcting the measured values or by using components with the smallest possible temperature drift for the circuit. However, this has the disadvantage of higher costs.
On account of the real properties of the analog integrator circuit, the measuring transducer signal or the Rogowski transducer voltage signal, which corresponds to the derivative of the current, is not completely or ideally transformed back or integrated. This produces a frequency-dependent phase shift, that is to say the phase is distorted. This can result in signal waveform distortion in mixed signals.
A phase error between the current and the voltage, which is produced on account of this analog integration, often cannot be completely eliminated. Consequently, current signals which have been shifted in this manner then result, for example, in incorrect measured power values, for example with regard to the active power.
The problem of analog integration has previously been solved by way of complicated calibration, components with nominal tolerances which are as tight as possible and with little temperature drift as well as complicated correction methods.